Charge-coupled device (CCD) image sensors are divided into an array of pixels by vertical p-implant channel barriers and by horizontal polysilicon gates with voltage applied. Full-frame devices use the same area for accumulation of photo-electron charge and for readout and therefore require a shutter to operate without smear or streaking. Interline and frame-transfer devices, on the other hand, provide regions specialized for these two functions.
Interline devices split each pixel region into a photodiode charge accumulation region and a CCD charge transfer region with a gate separating the two. Exposure and readout can then occur simultaneously, with charge accumulation proceding in the photodiode while the CCD portion of the pixel is transferring charge from the previous exposure vertically towards the output structures. When both the exposure and the readout are complete, it is possible to transfer the charge from the photodiode to the CCD portion of the pixel, whereupon accumulation of the next pixel can begin in the photodiode and readout of the just-transferred charge can begin in the CCD portion. The CCD portion is masked so that newly incoming light will not contaminate the image of the previous frame while it is being read out.
Frame transfer devices are divided into two regions of pixels with one region masked. During an accumulation and readout phase, image charge accumulates in the unmasked region while the previous image, stored in the masked region is read out. During a rapid transfer phase, the next accumulated frame is moved into the just-cleared masked region. In both cases, because transfer from accumulation region to readout region is fast compared with the total exposure and readout time, no shutter is necessary, the illumination is left on continuously and close to 100% duty cycle is possible.
In interline CCDs, the masking of light is not perfect. As shown in FIG. 1, high-angle incoming light 40 entering the photodiode and/or light which scatters into a high angle 45 by the photodiode 20 can go under the mask 15 and be detected in the CCD portion 10 of the pixel. Charge transfer direction is shown in FIG. 1 as in and out of the page. Because the image in the CCD portion is being moved vertically towards the output structures during readout, illumination which leaks into the CCD during readout will create a streak in the read-out image. FIG. 2 shows leakage 60 from a bright illumination source 50 into the masked charge-transfer structure 70, which can occur both during previous read or clear of the imaging area and after exposure during readout. The white cells 95 represent photodiodes. The charge-transfer cells in the top portion of the columns 70 have received leaked light during readout 60. The charge-transfer cells in the lower portion of the columns 90 have received leaked light during a clear or a previous read. Adjacent charge transfer cells shown in columns labeled 100 are depicted as not receiving any light from the depicted source 50.
In frame-transfer CCDs, the rapid transfer of the image from the unmasked, accumulation region to the masked readout region is not instantaneous but takes some time. When the illumination is on during the rapid scan a streak can again be generated since new light arriving during the rapid scan is added to different parts of the image as it is moved towards the readout structures. FIG. 3 shows an example of the effect 320 of charge leakage during readout scanning on an interline CCD-based electron microscope camera by leakage of light from the strong central spot 310 of the diffraction pattern.
As shown in FIG. 4, a frame transfer CCD, having an unmasked area 120 and a masked area 130 has a streak 111, 112, 113 and 114 due to the finite time taken to transfer the image under the storage-area mask. Analogous to the case of the interline CCD, there is a streak from scanning both before 114 and after exposure 112. The transferred charge from the image spot 110 is shown as 113. The streak from the current frame transfer is shown as 112 and the streak from the previous frame transfer is shown as 114.
Aside from interline and frame-transfer CCDs, there is sometimes a need to operate conventional full-frame CCDs without a shutter, as, for instance, when the frame rate is too high for the speed of a slow shutter. In this case, since the scan speed is normal slow readout and since the scanned image is not masked, a large component of smear can be added to the image as it is scanned and read.
It would be advantageous if there were a way to correct for streaking using digital post-processing. Imaging analysis alone is inadequate for this task, however, because while the streaking pattern is purely vertical in nature, the streak is underdetermined by the information in the streaked image alone because of the possibility of the existence of vertical or constant features in the image which could be incorrectly analyzed as streaking and removed from the image, creating artifacts. What is needed is a way to accurately measure the streaking independent of the exposed, accumulated image.
Thus there remains a need for a smear correction method which is accurate but which also causes only a modest reduction in frame rate and maintains fast response time to movements in the impinging image illumination.